1. Field of the Invention
The present invention relates to an on-line measurement of the amount of contaminant particles in lubricating oil, more particularly, to an apparatus which measures the amount of paramagnetic and non-magnetic contaminant particles in lubricating oil.
2. Description of the Prior Art
Due to the recent epochal development of technology in many industrial fields, technologies for preventive maintenance and monitoring the optimum conditions of machinery are widely needed to prevent an unexpected failure and an accident resulting from such failure in a system ranging from a vehicle to a large plant such as a power plant, an iron mill or a petrochemical plant.
Condition diagnosis technologies which has been put to practical use up to date can be classified based on a measurement method used therein into: a method for measuring variables used for system operation, e.g., temperature, pressure, and velocity; a method for measuring physical or chemical changes of mechanical parts or lubricating oil; a method for measuring energy loss due to friction or loss of material from abrasion. The technologies can also be classified based on a measurement period into an off-line method by which periodical checkup is made and an on-line method by which real time checkup is possible.
The present invention is related to a system for qualitatively measuring contamination level in lubricating oil. These kinds of technologies are the most important among various condition diagnosis technologies because by using them one can observe and detect the degree of damage, if any, of parts in a machine under operation without disassembling the machine at any time or in a continuous manner.
A condition diagnosis technique through a measurement of contamination level is to determine the health of a mechanical system, i.e., whether the system is damaged or in a normal condition, by measuring and analyzing the quantity of contaminant particles in lubricating oil used in the system which are produced by the wear in the system or provided from the exterior of the system. This technique is also called "machine health diagnosis" because it is similar to the technique for monitoring the health condition of a human body by qualitatively measuring the number of white or red blood cells.
In the condition diagnosis technologies using the measurement of the contaminant level, qualitative and/or quantitative measurements are made. As the quantitative measurement, the amount of wear debris is measured which is produced from machine elements as they are used. As the qualitative measurement, constituents of the wear debris are analyzed, changes of the particle size of the wear debris are measured, it is determined whether the contaminant has been introduced from the exterior of the system, and the degree of corrosion or oxidation of the machine parts is measured. Through these measurements and analyses, it is first determined whether the mechanical system is in a proper condition. Moreover, the analysis of changing characteristics of the wear debris can give early warning of impending failure. Also, one can understand major causes of the wear in the system from prolonged analysis of the wear characteristics. Result of such analysis can also be used in redesigning an improved system.
There have been many condition monitoring technologies by which one can qualitatively measure the contamination level in lubricating oil. Spectroscopy and a particle counter are examples of off-line condition monitoring techniques where oil samples are analyzed in a laboratory.
The spectroscopic apparatus, which determines the constituents of contaminant and their amount in lubricating oil, is currently used, e.g., to diagnose the condition of an airplane engine. Although it can precisely measure the contaminant level, it cannot give information on the size of contaminant particles and can only measure and analyze particles ranging in size from 1 to 20 .mu.m.
The particle counter is an apparatus for precisely measuring the size of contaminant particles, the distribution of the size, and total contaminant level. It has its own drawbacks in that it cannot be used for oil with high contaminant level and that it cannot give any information on the constituents of the contaminant particles.
Other condition monitoring techniques include a ferrographic apparatus disclosed in U.S. Pat. No. 4,187,170, a Rotary Particle Depositor (RPD) disclosed in U.K. Pat. No. 8,121,183 and a Particle Quantifier (PQ). In the ferrography and the RPD, contaminant particles are fixed to a surface of a transparent glass according to their sizes and the fixed particles are magnified and visualized by using an optical or electric microscope, so that the size, shape and color of the particles are analyzed. By analyzing the characteristics of the contaminant particles, we can identify the reason why the contaminant has been introduced to the oil and get information on the current condition of the system. However, in these techniques, the analysis results can be varied depending on the subjective view of the analyzer.
The above off-line methods for measuring and analyzing the contaminant level of the lubricating oil in a laboratory are inconvenient in that oil samples should be taken and carried to the laboratory periodically. During such sampling process, errors may occur. Moreover, by these off-line methods, one cannot detect a sudden failure of machinery and therefore an accident resulting from it cannot be prevented.
Examples of conventional apparatus for real-time measuring of contaminant level are a Quantitative Debris Monitor (QDM), a Magnetic Chip Detector and a Fluid Condition Monitor (FCM).
The QDM is a device for measuring contaminant level by magnetically inducing contaminant particles, making the induced particles collide with a surface of a sensor, and then observing voltage pulses resulting from such collision. Particles that can be measured by the QDM is limited to those over 100 .mu.m. In addition, since the surface of the sensor is prone to damage, the sensor should be frequently replaced.
In the magnetic chip detector, magnetic material is inserted in the lubricating oil so that paramagnetic contaminant particles are adhered to surfaces of the magnetic material. Then, the magnetic material is periodically analyzed by, for example, human eyes. Its merit is that it can be easily used in an industrial field. However, the paramagnetic particles that can be adhered to the surface of the magnetic material are limited to those over 100 .mu.m and, moreover, it cannot provide information on nonmagnetic contaminant particles.
Korean Patent No. 150054 is directed to "Method and apparatus for on-line monitoring of wear debris in lubricating oil." In summary, it discloses measuring the quantity of particles in the oil by measuring the amount of light intercepted by the particles when the light passes through the oil. It also discloses a method for separately measuring contaminant level due to paramagnetic particles by measuring the amount of light after selectively locating permanent magnets above the oil container and thereby isolating paramagnetic particles in the oil from the light passages. The above method has several drawbacks. First, to induce the minute particles distributed in the oil towards the magnet against the resistance of the oil, very strong magnet should be used, thus making the apparatus bulky. Second, it is highly probable that the oil which is being used in a field contains air bubbles. Therefore, if such oil is directly used for contaminant measurement and light is used as a measuring medium, air bubbles can be misidentified as contaminant particles so that the measured contaminant level may be higher than a real value. Moreover, the above patent simply mentions "on-line" measurement in its title but does not specifically disclose how the contaminant is measured on-line. Third, although the light-attenuating effect of the oil may vary depending on the viscosity of the oil, only the light-blocking effect of opaque contaminant particles is considered with no regard to the viscosity. Therefore, in an on-line measurement system, in order not to be affected by the change of viscosity, a device for maintaining constant viscosity is necessary, or alternatively, the amount of light should be adjusted depending on the temperature.
From the above, we can identify a couple of advantageous features of an online contaminant measurement system. First, it is preferable to analyze the contaminant particles qualitatively as well as quantitatively. Major elements in machinery are made of both paramagnetic material such as iron and non-magnetic material such as aluminum and copper. Therefore, by separately measuring the amount of wear debris constituting both material, we may detect machine parts where severe wear occurs.
Second, to use an on-line measurement system in a field for a long time, the cost of maintenance and repair should be minimized. In conventional measurement systems, filters or electromagnetic coated films are generally used. In a system where difference of pressure or liquid flow before and after the filter is measured, contaminant particles remaining on the surface of the filter should be removed by letting the liquid flow reversely after the measurement. However, it is difficult to know whether the filter is washed clean and the filter should be replaced after it is used for a certain amount of time. The electromagnetic coated film should also be replaced since it is damaged by the measurement.